Semiconductor laser mounting with intact diffusion barrier layer

ABSTRACT

A first contact surface of a semiconductor laser chip can be formed to a target surface roughness selected to have a maximum peak to valley height that is substantially smaller than a barrier layer thickness. A barrier layer that includes a non-metallic, electrically-conducting compound and that has the barrier layer thickness can be applied to the first contact surface, and the semiconductor laser chip can be soldered to a carrier mounting along the first contact surface using a solder composition by heating the soldering composition to less than a threshold temperature at which dissolution of the barrier layer into the soldering composition occurs. Related systems, methods, articles of manufacture, and the like are also described.

CROSS-REFERENCE TO RELATED APPLICATION

The present application is a continuation of and claims the prioritybenefit of U.S. patent application Ser. No. 15/652,177, filed Jul. 17,2017, which is a continuation of U.S. Ser. No. 14/873,080, filed Oct. 1,2015, which is a divisional of U.S. Ser. No. 13/212,085, filed Aug. 17,2011. The present application is also related to co-owned U.S. patentapplication Ser. No. 13/026,921, filed on Feb. 14, 2011, and to co-ownedU.S. patent application Ser. No. 13/027,000, filed on Feb. 14, 2011. Thedisclosure of each application identified in this paragraph isincorporated by reference herein in its entirety.

TECHNICAL FIELD

The present disclosure relates to an amperometric sensor for determiningmeasurement values of a measurand representing a chlorine dioxidecontent of a measuring fluid.

The subject matter described herein relates to frequency stabilizationof semiconductor lasers, in particular to mounting techniques for suchlasers.

BACKGROUND

A tunable laser-based trace gas analyzer, such as for example a tunablediode laser absorption spectrometer (TDLAS) can employ a narrow linewidth (e.g. approximately single frequency) laser light source that istuned across a trace gas absorption frequency range of a target analytefor each measurement of a sample volume of gas. Ideally, the laser lightsource in such an analyzer exhibits no material change in the startingand ending frequency of successive laser scans under a constant laserinjection current and operating temperature. Additionally, long termstability of the frequency tuning rate of the laser as a function of thelaser injection current, over the scan range, and over repetitive scansover a prolonged period of service can also be desirable.

Depending on the operational wavelength, however, currently availabletunable laser sources (e.g. diode lasers and semiconductor lasers) cantypically exhibit a wavelength drift on the order of a few picometers(on the order of gigahertz) per day to fractions of picometers per day.A typical trace gas absorption band line width can in some instances beon the order of a fraction of a nanometer to microns. Thus, drift orother variations in the output intensity of the laser light source can,over time, introduce critical errors in identification andquantification of trace gas analytes, particularly in gas having one ormore background compounds whose absorption spectra might interfere withabsorption features of a target analyte.

SUMMARY

In one aspect of the present disclosure, a method for frequencystabilization of semiconductor lasers comprises forming a first contactsurface of a semiconductor single-frequency laser chip for a tunablelaser-based trace gas analyzer to a target surface roughness, applying ametallization layer comprising 600·10-10 m of titanium to the firstcontact surface, and applying a metallic diffusion barrier layer to thefirst contact surface over the metallization layer, the metallicdiffusion barrier layer including multiple layers of differingmaterials. The method further comprises applying a solder preparationlayer to the first contact surface subsequent to applying the metallicdiffusion barrier layer and prior to soldering, wherein the solderpreparation layer includes an approximately 2000 to 5000·10-10 mthickness of gold, and soldering the laser chip along the first contactsurface to a carrier mounting using a solder composition, wherein thesoldering includes melting the solder composition by heating the soldercomposition to less than a threshold temperature at which dissolution ofthe metallic diffusion barrier layer into the solder composition occurs,wherein subsequent to the soldering, the metallic diffusion barrierlayer remains contiguous and intact such that no direct contact occursbetween semiconductor materials of the laser chip and the soldercomposition, such that no direct path exists by which constituents ofany of the laser chip, the solder composition and the carrier mountingcan diffuse across the metallic diffusion barrier layer. In anembodiment, subsequent to the soldering, the solder composition hassubstantially temporally stable electrical and thermal conductivities.

In at least one embodiment, applying the metallic diffusion barrierlayer includes applying a first metallic diffusion barrier layer, thefirst metallic diffusion barrier layer comprising platinum, and applyinga second metallic diffusion barrier layer underlaying the first metallicdiffusion barrier layer, the second metallic diffusion barrier layerincludes palladium, nickel, tungsten, molybdenum, titanium, tantalum,zirconium, cerium, gadolinium, chromium, manganese, aluminum, beryllium,or yttrium. In at least one embodiment, the method further comprisesproviding the solder composition as at least one of a solder preformthat is substantially non-oxidized and a deposited layer that issubstantially non-oxidized. In an embodiment, the soldering comprisesmelting the solder composition under at least one of a reducingatmosphere and a non-oxidizing atmosphere. In a further embodiment, thesolder composition is selected from a group consisting of goldgermanium, gold silicon, gold tin, silver tin, silver tin copper, silvertine lead, silver tin lead indium, silver tin antimony, tin lead, lead,silver, silicon, germanium, tin, antimony, bismuth, indium, and copper.

In another embodiment, the forming of the first contact surface includespolishing the first contact surface to achieve the target surfaceroughness prior to applying the metallic diffusion barrier layer. In anembodiment, the target surface roughness is less than approximately100·10⁻¹⁰ m RMS, and/or the target surface roughness is less thanapproximately 40·10⁻¹⁰ m RMS. In a further embodiment, the thresholdtemperature is less than approximately 400° C. In yet anotherembodiment, the threshold temperature is less than approximately 370° C.In still another embodiment, the threshold temperature is less thanapproximately 340° C.

In at least one embodiment, the method further comprises applyinganother barrier layer to a second contact surface of the carriermounting, and soldering the laser chip to the carrier mounting along thesecond contact surface. In a further embodiment, the method furthercomprises applying a solder facilitation layer between the first contactsurface and a second contact surface on the carrier mounting prior tothe soldering, the solder facilitation layer including a metal that isnot a component of the solder preparation layer on either the firstcontact surface or the second contact surface. In such an embodiment,applying the solder facilitation layer includes at least one of placinga sheet of the metal between the first contact surface and the secondcontact surface prior to the soldering, and depositing a layer of themetal that is not a component of the solder composition onto one or bothof the first contact surface and the second contact surface prior to thesoldering. In another embodiment, the method further comprises matchinga first thermal expansion characteristic of the carrier mounting to asecond thermal expansion characteristic of the semiconductor laser chip.

Another aspect of the present disclosure includes a tunable laser-basedtrace gas analyzer comprising a semiconductor single-frequency laserchip including a first contact surface having a target surfaceroughness, a metallization layer of 600·10⁻¹⁰ m of titanium applied tothe first contact surface, a metallic diffusion barrier layer applied tothe metallization layer, wherein the metallic diffusion barrier layerincludes multiple layers of differing materials, and a solderpreparation layer applied to the metallic diffusion barrier layer,wherein the solder preparation layer includes an approximately 2000 to5000·10⁻¹⁰ m thickness of gold, and a carrier mounting to which thelaser chip is soldered along the first contact surface of the laser chipusing a solder composition, wherein the solder composition is heated toless than a threshold temperature at which dissolution of the metallicdiffusion barrier layer into the solder composition occurs, and whereinthe metallic diffusion barrier layer is contiguous such that the soldercomposition does not directly contact semiconductor materials of thelaser chip, such that there is no direct path by which constituents ofany of the laser chip, the solder composition and the carrier mountingcan diffuse across the metallic diffusion barrier layer.

In at one least one embodiment, the tunable laser-based trace gasanalyzer is a tunable diode laser absorption spectrometer. In anembodiment, the metallic diffusion barrier layer includes a firstmetallic diffusion barrier layer including platinum and a secondmetallic diffusion barrier layer underlaying the first metallicdiffusion barrier layer, the second metallic diffusion barrier layerincluding palladium, nickel, tungsten, molybdenum, titanium, tantalum,zirconium, cerium, gadolinium, chromium, manganese, aluminum, beryllium,or yttrium. In a further embodiment, the target surface roughness isless than approximately 100·10⁻¹⁰ m RMS, and/or the target surfaceroughness is less than approximately 40·10⁻¹⁰ m RMS.

In at least one embodiment, the tunable laser-based trace gas analyzerfurther comprises a solder facilitation layer between the first contactsurface and a second contact surface on the carrier mounting, the solderfacilitation layer including a metal that is not a component of thesolder preparation layer on either the first contact surface or thesecond contact surface. In another embodiment, the tunable laser-basedtrace gas analyzer further comprises another barrier layer applied to asecond contact surface of the carrier mounting, wherein the laser chipis soldered to the carrier mounting along the second contact surface.

The details of one or more variations of the subject matter describedherein are set forth in the accompanying drawings and the descriptionbelow. Other features and advantages of the subject matter describedherein will be apparent from the description and drawings, and from theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and constitute apart of this specification, show certain aspects of the subject matterdisclosed herein and, together with the description, help explain one ormore features or the principles associated with the disclosedimplementations. In the drawings:

FIG. 1 is a graph illustrating effects of laser drift on performance ofa laser absorption spectrometer;

FIG. 2 is a second graph illustrating additional effects of laser drifton performance of a laser absorption spectrometer;

FIG. 3 is a schematic diagram illustrating a semiconductor laser chipsecured to a carrier mount;

FIG. 4 is a process flow diagram illustrating aspects of a method havingone or more features consistent with implementations of the currentsubject matter;

FIG. 5 is a diagram showing an end elevation view of a conventionalTO-can mount such as are typically used for mounting semiconductor laserchips;

FIG. 6 is a diagram showing a magnified view of a carrier mount and asemiconductor laser chip affixed thereto;

FIG. 7 is a scanning electron micrograph showing a solder joint betweena semiconductor laser chip and a carrier mount;

FIG. 8 is a chart showing a phosphorous concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 9 is a chart showing a nickel concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 10 is a chart showing an indium concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 11 is a chart showing a tin concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 12 is a chart showing a lead concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7;

FIG. 13 is a chart showing a tungsten concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7; and

FIG. 14 is a chart showing a gold concentration measured by X-raydiffraction as a function of depth in the apparatus shown in FIG. 7.

When practical, similar reference numbers denote similar structures,features, or elements.

DETAILED DESCRIPTION

Experimental data have revealed that laser emission wavelength changesas small as 1 picometer (pm) or less between spectral scans in a laserabsorption spectrometer using a scannable or tunable laser source canmaterially alter a trace gas concentration determination with respect toa measurements obtainable with a spectral analyzer in its originalcalibration state. An example of spectral laser spectroscopy using adifferential spectroscopy approach is described in co-owned U.S. Pat.No. 7,704,301, the disclosure of which is incorporated herein in itsentirety. Other experimental data have indicated that a tunable diodelaser-based analyzer designed for low analyte concentration detectionand quantification (e.g. on the order of parts per million (ppm) ofhydrogen sulfide (H₂S) in natural gas) and employing a harmonic (e.g.2f) wavelength modulation spectral subtraction approach can unacceptablydeviate from its calibration state due to a shift in laser output of assmall as 20 picometers (pm) at constant injection current and constanttemperature (e.g. as controlled by a thermoelectric cooler).

In general terms, a laser frequency shift that can be acceptable formaintaining an analyzer calibration within its accuracy specificationdrops with smaller target analyte concentrations and also withincreasing spectral interferences from other components of a samplemixture at the location of the target analyte absorption. Formeasurements of higher levels of a target analyte in a substantiallynon-absorbing background, larger laser frequency shifts can be toleratedwhile maintaining the analyzer calibration state.

The graphs 100 and 200 shown in FIG. 1 and FIG. 2, respectively, showexperimental data illustrating potential negative impacts of laseroutput variations that may be caused by changes in characteristics (e.g.physical, chemical, and the like) of a semiconductor laser source overtime. The reference curve 102 shown in the graph 100 of FIG. 1 wasobtained with a tunable diode laser spectrometer for a reference gasmixture containing approximately 25% ethane and 75% ethylene. The testcurve 104 was obtained using the same spectrometer after some time hadpassed for a test gas mixture containing 1 ppm acetylene in a backgroundof approximately 25% ethane and 75% ethylene. Acetylene has a spectralabsorption feature in the range of about 300 to 400 on the wavelengthaxis of the chart 100 in FIG. 1. If the spectrometer were not adjustedin some manner to compensate for the drift observed in the test curve104 relative to the reference curve 102, the measured concentration ofacetylene from the spectrometer would be, for example, −0.29 ppm insteadof the correct value of 1 ppm.

Similarly, in FIG. 2, the chart 200 shows a reference curve 202 obtainedwith a tunable diode laser spectrometer for a reference gas mixturecontaining approximately 25% ethane and 75% ethylene. The test curve 204was obtained for a test gas mixture containing 1 ppm acetylene in abackground of approximately 25% ethane and 75% ethylene. As shown inFIG. 2, the line shape of the test curve 204 is distorted relative tothe line shape of the reference curve 202 due to drift or other factorsaffecting performance of the laser absorption spectrometer over time. Ifthe test curve 204 were not corrected to compensate for the distortionobserved in the test curve 204 relative to the reference curve 202, themeasured concentration of acetylene in the test gas mixture determinedby the spectrometer would be, for example, 1.81 ppm instead of the trueconcentration of 1 ppm.

Based on Ohm's Law (i.e. P=I2R where P is the power, I is the current,and R is the resistance), a current-driven semiconductor laser chip willgenerate waste heat that increases approximately as the square of theinjection current driving the laser. While the resistance, R, of thesemiconductor diode laser assembly is not typically linear or constantwith changes in temperature, an approximately quadratic increase inwaste heat with increases in current is generally representative ofreal-world performance. Thermal roll-over, in which the power output ofa laser is reduced at excessive temperatures, can typically occurbecause the lasing efficiency of a typical band-gap type directsemiconductor laser diode decreases with increasing p-n junctionoperating temperature. This is especially true for infrared lasers, suchas for example lasers based on indium phosphide (InP) or galliumantimonide (GaSb) material systems.

Single frequency operation of an infrared semiconductor laser can beachieved by employing DFB (distributed feedback) schemes, whichtypically use optical gratings, either incorporated in the laser ridgeof the semiconductor laser crystal in the form of semiconductor crystalindex of refraction periodicities or placed laterally to the laser ridgeas metal bars. The effective optical periods of the approaches of thevarious gratings determining the laser emission wavelength can typicallydepend upon the physical spacing of the metal bars of the grating orupon the physical dimension of the ridge-regrown semiconductor materialzones with different index of refraction and the index of refraction ofthe respective semiconductor materials. In other words, the emissionwavelength of a semiconductor laser diode, such as are typically usedfor tunable diode laser spectroscopy, can depend primarily upon thelaser p-n junction and on the laser crystal operating temperature andsecondarily on the carrier density inside the laser. The laser crystaltemperature can change the grating period as a function of thetemperature dependent thermal expansion of the laser crystal along itslong optical cavity axis and as a function of the temperature dependentindex of refraction.

Typical injection current-related and temperature-related wavelengthtuning rates of infrared lasers useable for tunable diode laser tracegas analyzers can be on the order of approximately 0.1 nanometers per °C. and approximately 0.1 nanometers per milli-ampere. As such, it can bedesirable to maintain semiconductor laser diodes for precision TDLASdevices at a constant operating temperature within a few thousandths ofa ° C. and at injection currents that are controlled to within a fewnano-amperes.

Long term maintenance and regeneration of a TDLAS calibration state andthe related long term measurement fidelity with respect to the originalinstrument calibration can require the ability to substantiallyreplicate the correct laser operating parameters in the wavelength spacefor any given measurement. This can be desirable for spectroscopytechniques employing subtraction of spectral traces (e.g. differentialspectroscopy), such as is described in co-owned U.S. Pat. No. 7,704,301;pending U.S. patent applications Ser. Nos. 13/027,000 and 13/026,091 and12/814,315; and U.S. Provisional Application No. 61/405,589, thedisclosures of which are incorporated by reference herein.

Commercially available single frequency semiconductor lasers that aresuitable for trace gas spectroscopy in the 700 nm to 3000 nm spectralrange have been found to generally exhibit a drift of their centerfrequency over time. Drift rates of several picometers (pm) to fractionsof a pm per day have been confirmed by performing actual molecular tracegas spectroscopy over periods of 10 days to more than 100 days. Lasersthat may behave as described can include, but are not limited to, laserslimited to single frequency operation by gratings etched into the laserridge (e.g. conventional telecommunications grade lasers), Bragggratings (e.g. vertical cavity surface emitting lasers or VCSELs),multiple layer narrow band dielectric mirrors, laterally coupledgratings, and the like. Frequency drift behavior has been observed withsemiconductor diode lasers; VCSELs; horizontal cavity surface emittinglasers HCSEL's (HCSELs); quantum cascade lasers built on semiconductormaterials including but not limited to indium phosphide (InP), galliumarsenide (GaAs), gallium antimonide (GaSb), gallium nitride (GaN),indium gallium arsenic phosphide (InGaAsP), indium gallium phosphide(InGaP), indium gallium nitride (InGaN), indium gallium arsenide(InGaAs), indium gallium aluminum phosphide (InGaAlP), indium aluminumgallium arsenide (InAlGaAs), indium gallium arsenide (InGaAs), and othersingle and multiple quantum well structures.

Approaches have been previously described to re-validate the performanceof a tunable laser. For example, as described in U.S. patent applicationSer. Nos. 13/026,921 and 13/027,000 referenced above, a referenceabsorption line shape collected during a calibrated state of an analyzercan be compared to a test absorption line shape collected subsequently.One or more operating parameters of the analyzer can be adjusted tocause the test absorption line shape to more closely resemble thereference absorption line shape.

Reduction of the underlying causes of frequency instability insemiconductor-based tunable lasers can also be desirable, at leastbecause compensation of laser shift and outputted line shapes tomaintain analyzer calibration by adjusting the semiconductor diode laseroperating temperature or the median drive current may only be possibleover limited wavelength shifts due to a typically non-linear correlationbetween injection current and frequency shift in semiconductor laserdiodes (e.g. because of thermal roll-over as discussed above). Thenonlinearity of the frequency shift as a function of injection currentmay change as a function of laser operating temperature set by atemperature control device (e.g. a thermoelectric cooler or TEC) and themedian injection current. At higher control temperatures, thermalroll-over may occur at lower injection currents while at lower controltemperatures, the roll-over may occur at higher injection currents.Because the control temperature and injection current combined determinethe laser emission wavelength, not all combinations of controltemperature and median injection current used to adjust the laserwavelength to the required target analyte absorption line will providethe same frequency scan and absorption spectra.

Accordingly, one or more implementations of the current subject matterrelate to methods, systems, articles or manufacture, and the like thatcan, among other possible advantages, provide semiconductor-based lasersthat have substantially improved wavelength stability characteristicsdue to a more temporally stable chemical composition of materials usedin affixing a semiconductor laser chip to a mounting device. Someimplementations of the current subject matter can provide or include asubstantially contiguous and intact diffusion barrier layer thatincludes at least one non-metallic layer and alternatively at least onenon-metallic and at least one metallic barrier layer at or near acontact surface between a semiconductor laser chip and a mountingsurface. Drift of single frequency lasers can be reduced or evenminimized, according to one or more implementations, by employingsemiconductor laser designs, laser processing, electrical connections,and heat sinking features that reduce changes in heat conductivity, instress and strain on the active laser, and in electrical resistivity ofthe injection current path over time.

FIG. 3 illustrates an example of an apparatus 300 including asemiconductor laser chip 302 affixed to a mounting device 304 by a layerof solder 306 interposed between a contact surface 310 of thesemiconductor laser chip 302 and the mounting device 304. The mountingdevice can function as a heat sink and can provide one or moreelectrical connections to the semiconductor laser chip 302. One or moreother electrical connections 312 can be provided to connect a p or njunction of the semiconductor laser chip 302 to a first polarity and theother junction to a second polarity, for example via conduction throughthe solder layer 306 into the carrier mount 304.

FIG. 4 shows a process flow chart illustrating a method includingfeatures that can be present in one or more implementations of thecurrent subject matter. At 402, a first contact surface of asemiconductor laser chip is formed to a target surface roughness. Thetarget surface roughness is selected to have a maximum peak to valleyheight that is substantially smaller than a barrier layer thickness of abarrier layer to be applied to the first contact surface. At 404, thatbarrier layer is applied to the first contact surface with the barrierlayer thickness. The barrier layer includes the at least onenon-metallic, electrically-conducting compound, examples of whichinclude but are not limited to titanium nitride (TiNX), titaniumoxy-nitride (TiNXOY), cerium gadolinium oxy-nitride (CeGdOyNX) ceriumoxide (CeO2), and tungsten nitride (WNx). At 406, the semiconductorlaser chip is soldered to a carrier mounting along the first contactsurface using a solder composition. The soldering includes melting thesoldering composition by heating the soldering composition to less thana threshold temperature at which dissolution of the barrier layer intothe soldering composition occurs.

In some implementations, a contact surface 310 of a laser semiconductorchip 302 can be polished or otherwise prepared to have a target surfaceroughness of less than approximately 100 Å RMS (root mean square), oralternatively of less than approximately 40 Å RMS. Conventionalapproaches have typically not focused on the surface roughness of thecontact surface 310 and have consequently had surface roughness valuesof greater than approximately 1 μm RMS. Subsequent to preparing asufficiently smooth contact surface 310, the contact surface 310 can betreated to form one or more barrier layers.

Creation of a barrier layer that can survive the soldering process canbe aided by polishing of the first contact surface 310 to a low surfaceroughness. In general, a total thickness of a metallic barrier layer,for example one made of platinum, may only be deposited at a limitedthickness due to very high stresses that can lead to a separation ofthicker layers from the semiconductor material of the semiconductorlaser chip 302. The barrier layer can include multiple layers ofdiffering materials. In an implementation, at least one of the barrierlayers can include a non-metallic, electrically conducting compound,such as for example titanium nitride (TiNX), titanium oxy-nitride(TiNXOY), cerium gadolinium oxy-nitride (CeGdyONX), cerium oxide (CeO2),and tungsten nitride (WNx). One or more additional barrier layersoverlaying or underlaying the first barrier layer can include a metalincluding but not limited to platinum (Pt), palladium (Pd), nickel (Ni),tungsten (W), molybdenum (Mo) titanium (Ti), tantalum (Ta), zirconium(Zr), cerium (Ce), gadolinium (Gd), chromium (Cr), manganese (Mn),aluminum (Al), beryllium (Be), and Yttrium (Y).

A solder composition can in some implementations be selected from acomposition having a liquidus temperature, i.e. the maximum temperatureat which solid crystals of an alloy can co-exist with the melt inthermodynamic equilibrium, of less than approximately 400° C., oroptionally of less than approximately 370° C., or optionally of lessthan approximately 340° C. Examples of solder compositions consistentwith one or more implementations of the current subject matter caninclude, but are not limited to gold germanium (AuGe), gold silicon(AuSi), gold tin (AuSn), silver tin (AgSn), silver tin copper (AgSnCu),silver tin lead (AgSnPb), silver tin lead indium (AgSnPbIn), silver tinantimony (AgSnSb), tin lead (SnPb), and lead (Pb). Examples of specificsolder compositions that are consistent with one or more implementationsof the current subject matter include, but are not limited to thefollowing: approximately 48% Sn and approximately 52% In; approximately3% Ag and approximately 97% In; approximately 58% Sn and approximately42% In; approximately 5% Ag, approximately 15% Pb, and approximately 80%In; approximately 100% In; approximately 30% Pb and approximately 70%In; approximately 2% Ag, approximately 36% Pb, and approximately 62% Sn;approximately 37.5% Pb, approximately 37.5% Sn, and approximately 25%In; approximately 37% Pb and approximately 63% Sn; approximately 40% Pband approximately 60% In; approximately 30% Pb and approximately 70% Sn;approximately 2.8% Ag, approximately 77.2% Sn, and approximately 20% In;approximately 40% Pb and approximately 60% Sn; approximately 20% Pb andapproximately 80% Sn; approximately 45% Pb and approximately 55% Sn;approximately 15% Pb and approximately 85% Sn; approximately 50% Pb andapproximately 50% In; approximately 10% Pb and approximately 90% Sn;approximately 10% Au and approximately 90% Sn; approximately 3.5% Ag andapproximately 96.5% Sn; approximately 60% Pb and approximately 40% In;approximately 3.5% Ag, approximately 95% Sn, and approximately 1.5% Sb;approximately 2.5% Ag and approximately 97.5% Sn; approximately 100% Sn;approximately 99% Sn and approximately 1% Sb; approximately 60% Pb andapproximately 40% Sn; approximately 97% Sn and approximately 3% Sb;approximately 95% Sn and approximately 5% Sb; approximately 63.2% Pb,approximately 35% Sn, and approximately 1.8% In; approximately 70% Pband approximately 30% Sn; approximately 75% Pb and approximately 25% In;approximately 80% Pb approximately 20% Sn; approximately 81% Pb andapproximately 19% In; approximately 80% Au and approximately 20% Sn;approximately 86% Pb, approximately 8% Bi, approximately 4% Sn, andapproximately 1% In, approximately 1% Ag; approximately 85% Pb andapproximately 15% Sn; approximately 2% Ag, approximately 88% Pb, andapproximately 10% Sn; approximately 5% Ag, approximately 90% Pb, andapproximately 5% Sn; approximately 95% Pb and approximately 5% Sb;approximately 2.5% Ag, approximately 92.5% Pb, and approximately 5% Sn;approximately 2.5% Ag, approximately 92.5% Pb, and approximately 5% In;approximately 90% Pb and approximately 10% Sn; approximately 2.5% Ag andapproximately 97.5% Pb; approximately 2.5% Ag; approximately 95.5% Pb,and approximately 2% Sn; approximately 78% Au and approximately 22% Sn;approximately 1.5% Ag, approximately 97.5% Pb, and approximately 1% Sn;approximately 5% Ag, approximately 90% Pb, and approximately 5% In;approximately 95% Pb and approximately 5% In; and approximately 95% Pband approximately 5% Sn.

FIG. 5 shows an end elevation view of a conventional transistor outlinecan (TO-can) mount 500 such as is typically used in mounting ofsemiconductor laser chips for use in telecommunications applications.TO-cans are widely used electronics and optics packaging platforms usedfor mechanically mounting, electrically connecting, and heat sinkingsemiconductor chips such as lasers and transistors and are available ina variety of different sizes and configurations. An outer body 502encloses a post or heat sink member 504 which can be made of metal, suchas for example a copper tungsten sintered metal, copper-diamond sinteredmetal, or iron-nickel alloys including Kovar, alloy 42, and alloy 52.Two insulated electrical pass-throughs 506 can be included to provideelectrical contacts for connection to the p and n junctions on asemiconductor laser chip 302. The semiconductor laser chip 302 can bemounted to a carrier sub-mount, which can in some examples be formed ofsilicon. As noted above, the semiconductor laser chip 302 can be joinedto the carrier mount 304 (also referred to as a carrier mounting) by alayer of solder 306, which is not shown in FIG. 5 due to scaleconstraints. FIG. 6 shows a magnified view 600 of the post or heat sinkmember 504, the carrier mount 304, the semiconductor laser chip 302, andthe solder 306 joining the semiconductor laser chip 302 to the carriermount. The carrier mount 304 can in turn be soldered to the post or heatsink member 504 by a second solder layer 602.

According to one or more implementations of the current subject matter,mono-component layers (or surfaces) of a material dissimilar from thesolder preparation layer, which includes but is not limited to gold, canserve similarly as solder alloys, enabling low temperature joining oftwo gold surfaces for instance. In joining metal components, this iscommonly referred to as liquidus or liquid diffusion bonding since itapparently creates a very thin liquid interface layer between certaindissimilar metals which are brought in physical contact under elevatedtemperature. The temperature necessary to cause this effect to occur istypically significantly lower than the component melting temperatures.Once the initial joining has taken place, thermal separation cantypically require quite high temperatures, approaching or reaching thecomponent melting temperatures. In one example, contact between a silversurface and a gold surface can result in a hermetic joint at atemperature of approximately 150° C. to approximately 400° C., which issignificantly lower than the separate melting temperatures of silver(950° C.) and gold (1064° C.). In another example, copper oxide canserve as a bonding promotion layer which can reduce a joiningtemperature between two metal surfaces significantly below the metalmelting points.

Thus, in some implementations, solder compositions including lead (Pb),silver (Ag), silicon (Si), germanium (Ge), tin (Sn), antimony (Sb),bismuth (Bi), indium (In), and copper (Cu) as well as those discussedelsewhere herein can be used in association with deposition of one ormore solder-facilitating mono-component material layers or thin sheets(e.g. preforms) on the first contact surface 310 and/or second contactsurface 314 prior to the soldering process. The one or moresolder-facilitating mono-component material layers or thin sheets can bedissimilar from other barrier and/or metallization layers on the firstcontact surface 310 and/or second contact surface 314. One example of amethod for applying solder-facilitating mono-component material layersor thin sheets can include depositing a thin layer of a metal differingfrom those metals present in the solder preparation layer on top of thebarrier and/or metallization layers on the first contact surface 310and/or second contact surface 314. Such a thin layer can be evaporatedor otherwise deposited onto one or both of the first contact surface 310and the second contact surface 314 shortly before the heat assistedjoining process takes place, in order to prevent or minimize oxidation.Alternately, a thin sheet of a metal dissimilar from the solderpreparation layer can be placed between the semiconductor laser chip 302and the mounting device 304. The soldering process can then proceed asdiscussed above.

FIG. 7 shows an electron micrograph 700 showing a highly magnifiedsolder layer 306 interposed between a semiconductor laser chip 302 and acarrier mount 304. A second barrier layer 702 of nickel is also providedon the second contact surface 704 of the carrier mount 304. A verticalaxis 706 is displayed atop the electron micrograph to delineate distancefrom an arbitrarily chosen origin coordinate (marked as “0” on the axis706) to a linear distance of 50 microns away (marked as “50” on the axis706). The semiconductor laser chip 302 shown in FIG. 7 was not preparedwith a smooth first contact surface 310 as described herein consistentaccording various implementations of the current subject matter. As aresult, the first contact surface 310 exhibits substantial surfaceroughness, and no contiguous barrier layer remains to separate thematerial of the semiconductor laser chip 302 from the solder after thesoldering process. FIG. 8 through FIG. 14 show a series of charts 800,900, 1000, 1100, 1200, 1300, and 1400 showing relative concentrations ofphosphorous, nickel, indium, tin, lead, tungsten, and gold,respectively, as a function of distance along the axis 706 in FIG. 7.The relative concentrations were determined by an X-ray diffractiontechnique.

As shown in the chart 800 of FIG. 8, a large phosphorous concentrationis observed in the semiconductor laser chip 302 (distance greater thanabout 36 μm) due to the semiconductor laser chip 302 being a crystal ofindium phosphide (InP). Additional high relative concentrations ofphosphorous are observed in the nickel barrier layer 702, which isactually formed of a first layer 710 of nickel deposited by anelectroless process that incorporates some phosphorous into thedeposited nickel and a second layer of nickel deposited by anelectrolytic process that incorporates less or no phosphorous into thedeposited nickel. A non-zero concentration of phosphorous occurs both inthe solder (which is composed of a tin-lead alloy and does not containany phosphorus in its original state) and in the electrolytic secondlayer 712 of nickel. These non-zero concentrations are respectively dueto diffusion of phosphorous from the crystal structure of thesemiconductor laser chip 302 and from the electroless first layer 710 ofnickel.

FIG. 9 illustrates that some nickel also diffuses into the solder 306from the nickel layer 702 and further into the crystal structure of thesemiconductor laser chip 302. Similarly, indium diffuses into the solder306 and from there into the carrier mount across the nickel barrierlayer 702 as shown in the chart 1000 of FIG. 10. Tin, which is a primarycomponent of the solder 306, does not remain in the solder 306, but alsodiffuses into the crystal structure of the semiconductor laser chip 302as shown in the chat 1100 of FIG. 11. Lead also diffuses out of thesolder layer 306 as shown in the chart 1200 of FIG. 12, but to a lesserdegree than does the tin from the solder 306. Tungsten from thetungsten-copper carrier mount 304 and gold from solder preparationlayers deposited on both of the first contact surface 310 and the secondcontact surface 702 diffuse into the solder and to a small extent intothe semiconductor laser chip 302 as shown in the charts 1300 and 1400 ofFIG. 13 and FIG. 14.

Accordingly, features of the current subject matter that allow themaintenance of a contiguous, intact barrier layer at least at the firstcontact surface 310 of the semiconductor laser chip 302, and alsodesirably at the second contact surface 704 of the carrier mount 304 canbe advantageous in minimizing diffusion of elements from the carriermount and/or semiconductor laser chip across the barrier layer and canthereby aid in maintaining a more temporally consistent composition ofboth the solder layer 306 and the crystal structure of the semiconductorlaser chip 302. The presence of phosphorous and/or other reactivecompounds or elements, such as for example oxygen, antimony, silicon,iron and the like in the solder layer 306 can increase a tendency of thesolder alloy components to react and thereby change in chemicalcomposition, in crystal structure, hermeticity and, more importantly, inelectrical and/or thermal conductivity. Such changes can lead toalteration in the laser emission characteristics of a semiconductorlaser chip 302 in contact with the solder layer 306.

Furthermore, diffusion of solder components, such as for example lead;silver; tin; and the like; and/or carrier mount components such astungsten, nickel, iron, copper and the like, into the crystal structureof the semiconductor laser chip 302 can also cause changes in the laseremission characteristics over time.

Implementations of the current subject matter can provide one or moreadvantages, including but not limited to maintaining a contiguousdiffusion barrier layer between a laser crystal or other semiconductorchip and its physical mounting, preventing inter-diffusion of soldercompounds into the laser crystal and vice versa, and preventingcontamination of the solder. Inter-diffusion and/or electro-migrationhave been found to cause changes in the electrical resistivity, and to alesser extent the heat conduction properties, of the contact. Very smallchanges in resistive heating of even one of the electrical contactsproviding a driving current to a semiconductor laser chip can lead tofrequency changes in the light produced by the semiconductor laser chip.

In some observed examples using conventional semiconductor laser chipmounting approaches, induced shifts in the laser output can be greaterthan a picometer per day. Implementations of the current subject mattercan therefore include one or more techniques for improving barrierlayers at one or more of the first contact surface 310 between thesolder layer 306 and the semiconductor laser chip 302 and the secondcontact layer 702 between the solder layer 306 and the carrier mount304. In one example, an improved barrier layer at the second contactsurface 702 can include an electroless plated nickel underlayer 710, forexample to preserve edge definition of a copper tungsten submount or thelike, covered by a minimum thickness of an electrolytic nickel layer 712as the final layer before deposition of a gold solder preparation layer.In another example, a single layer of a sputtered barrier material,including but not limited to at least one of nickel, platinum,palladium, and electrically conducting non-metallic barrier layers, canbe used as a single barrier layer at the first contact surface 310. Asoxidation of the solder material prior to soldering of the semiconductorlaser chip 302 to the carrier mount 304 can introduce oxygen and otherpotentially reactive contaminants, it can be advantageous to use solderforms that have not been allowed to substantially oxidize prior to use.Alternatively, the soldering process can be performed under anon-oxidizing atmosphere or under a reducing atmosphere including butnot limited to vacuum, pure nitrogen pure hydrogen gas (H₂), forming gas(approximately 5% hydrogen in 95% nitrogen), and formic acid in nitrogencarrier gas to remove or at least reduce the presence of oxidizedcompounds in the solder composition on the metalized semiconductorcontact surface and the carrier mounting surface.

Suitable barrier layers to be deposited on the first contact surface 310and/or the second contact surface 702 can include, but are not limitedto, platinum (Pt), palladium (Pd), nickel (Ni), titanium nitride(TiN_(X)), titanium oxy-nitride (TiN_(X)O_(Y)), tungsten nitride(WN_(x)), cerium oxide (CeO₂), and cerium gadolinium oxy-nitride(CEGDO_(Y)N_(X)). These compounds, as well as other comparable compoundsthat can be deposited by sputtering or vapor deposition onto the firstand/or second contact surfaces, can provide a barrier layer that has asufficiently high temperature resistance during the soldering process asto not dissolve in the solder or otherwise degrade sufficiently to causebreakdown of the barrier qualities necessary to prevent cross-barrierdiffusion of semiconductor laser materials into the solder or soldercomponents into the semiconductor laser crystal. The second barrierlayer 702 applied to the second contact surface 704 can in someimplementations include a sintered diamond-copper layer. A process forcreation of a non-metallic, electrically-conducting barrier layer 702can include first depositing titanium via a thin film depositionprocess, including but not limited to sputtering, electron beamevaporation, chemical vapor deposition, atomic layer deposition, and thelike, and then adding nitrogen to react with the deposited titanium. Inanother implementation, a first metallization layer can be deposited bya thin film deposition process, and nitrogen ions can be used forsputtering titanium, for example in a nitrogen gas background, to createthe non-metallic barrier layer. Chemical vapor deposition can also oralternatively be used to create non-metallic barrier layers. In anotherimplementation, gas phase reactions of the components elements orcompounds forming the non-metallic electrically conductive compound canbe used to create multi-component non-metallic barrier layers.

In some implementations, the heat conductivity of a carrier mount 304can advantageously exceed 50 Watts per meter-Kelvin or, optionally 100Watts per meter-Kelvin or, optionally 150 Watts per meter-Kelvin.Suitable carrier mount materials can include, but are not limited tocopper tungsten, tungsten, copper-diamond, aluminum nitride, silicon,silicon nitride, silicon carbide, beryllium oxide, alumina (Al2O3),Kovar, Alloy 42, Alloy 52, and the like. A heat spreader or carriermount 304 that is thermally expansion matched to the semiconductor laserchip 302 can be used in some implementations. In one example consistentwith an implementation of the current subject matter, an approximately15% copper, approximately 85% tungsten sintered metal heat spreader, aberyllium oxide heat spreader, an alumina heat spreader, a sapphire heatspreader, a copper-diamond heat spreader, or the like can provide a goodthermal expansion match to a gallium antimonide (GaSb) semiconductorlaser chip 302 at around approximately 7 ppm° C.-1. In another exampleconsistent with an implementation of the current subject matter, a puretungsten heat spreader, a silicon heat spreader, a silicon nitride heatspreader, a silicon carbide heat spreader, a sapphire heat spreader, acopper diamond heat spreader, or an aluminum nitride (AlN) heat spreadercan be used as a carrier mount 304 to provide a good thermal expansionmatch to an indium phosphide (InP) semiconductor laser chip 302 ataround 4.5 ppm° C.-1. A silicon, silicon carbide, silicon nitride,aluminum nitride, tungsten, copper diamond heat spreader, or the likecan also be used as the carrier sub-mount 304, for example for an indiumphosphide (InP) semiconductor laser chip 302.

Other carrier mounts consistent with implementations of the currentsubject matter include, but are not limited to shaped copper tungstenheat spreaders, including but not limited to semiconductor laserindustry standard c-mounts and CT-mounts, TO-cans, pattern metallizedceramics, pattern metallized silicon, pattern metallized siliconcarbide, pattern metallized silicon nitride, pattern metallizedberyllium oxide, pattern metallized alumina, pattern metallized aluminumnitride, copper-diamond, pure copper with one or more sections ofexpansion-matched submounts to match to one or more semiconductor laserchip compositions, tungsten submounts brazed into a copper or coppertungsten c-mount, or the like. Semiconductor laser chips 302 can beformed, without limitation of indium phosphide crystals, galliumarsenide crystals, gallium antimonide crystals, gallium nitridecrystals, and the like.

The subject matter described herein can be embodied in systems,apparatus, methods, and/or articles depending on the desiredconfiguration. The implementations set forth in the foregoingdescription do not represent all implementations consistent with thesubject matter described herein. Instead, they are merely some examplesconsistent with aspects related to the described subject matter.Although a few variations have been described in detail above, othermodifications or additions are possible. In particular, further featuresand/or variations can be provided in addition to those set forth herein.For example, the implementations described above can be directed tovarious combinations and subcombinations of the disclosed featuresand/or combinations and subcombinations of several further featuresdisclosed above. In addition, the logic flows depicted in theaccompanying figures and/or described herein do not necessarily requirethe particular order shown, or sequential order, to achieve desirableresults. Other implementations may be within the scope of the followingclaims.

Claimed is:
 1. A method for frequency stabilization of semiconductorlasers comprising: forming a first contact surface of a semiconductorsingle-frequency laser chip for a tunable laser-based trace gas analyzerto a target surface roughness; applying a metallization layer comprising600·10⁻¹⁰ m of titanium to the first contact surface; applying ametallic diffusion barrier layer to the first contact surface over themetallization layer, the metallic diffusion barrier layer includingmultiple layers of differing materials, wherein applying the metallicdiffusion barrier layer includes: applying a first metallic diffusionbarrier layer, the first metallic diffusion barrier layer comprisingplatinum; and applying a second metallic diffusion barrier layerunderlaying the first metallic diffusion barrier layer, the secondmetallic diffusion barrier layer includes palladium, nickel, tungsten,molybdenum, titanium, tantalum, zirconium, cerium, gadolinium, chromium,manganese, aluminum, beryllium, or yttrium; applying a solderpreparation layer to the first contact surface subsequent to applyingthe metallic diffusion barrier layer and prior to soldering, wherein thesolder preparation layer includes an approximately 2000 to 5000·10⁻¹⁰ mthickness of gold; and soldering the laser chip along the first contactsurface to a carrier mounting using a solder composition, wherein thesoldering includes melting the solder composition by heating the soldercomposition to less than a threshold temperature at which dissolution ofthe metallic diffusion barrier layer into the solder composition occurs,wherein subsequent to the soldering, the metallic diffusion barrierlayer remains contiguous and intact such that no direct contact occursbetween semiconductor materials of the laser chip and the soldercomposition, such that no direct path exists by which constituents ofany of the laser chip, the solder composition and the carrier mountingcan diffuse across the metallic diffusion barrier layer.
 2. The methodof claim 1, wherein, subsequent to the soldering, the solder compositionhas substantially temporally stable electrical and thermalconductivities.
 3. The method of claim 1, further comprising providingthe solder composition as at least one of a solder preform that issubstantially non-oxidized and a deposited layer that is substantiallynon-oxidized.
 4. The method of claim 1, wherein the soldering includesmelting the solder composition under at least one of a reducingatmosphere and a non-oxidizing atmosphere.
 5. The method of claim 1,wherein the solder composition is selected from a group consisting ofgold germanium, gold silicon, gold tin, silver tin, silver tin copper,silver tine lead, silver tin lead indium, silver tin antimony, tin lead,lead, silver, silicon, germanium, tin, antimony, bismuth, indium, andcopper.
 6. The method of claim 1, wherein the forming of the firstcontact surface includes polishing the first contact surface to achievethe target surface roughness prior to applying the metallic diffusionbarrier layer.
 7. The method of claim 6, wherein the target surfaceroughness is less than approximately 100·10⁻¹⁰ m RMS, and/or wherein thetarget surface roughness is less than approximately 40·10⁻¹⁰ m RMS. 8.The method of claim 1, wherein the threshold temperature is less thanapproximately 400° C.
 9. The method of claim 8, wherein the thresholdtemperature is less than approximately 370° C.
 10. The method of claim9, wherein the threshold temperature is less than approximately 340° C.11. The method of claim 1, further comprising applying another barrierlayer to a second contact surface of the carrier mounting, and solderingthe laser chip to the carrier mounting along the second contact surface.12. The method of claim 1, further comprising applying a solderfacilitation layer between the first contact surface and a secondcontact surface on the carrier mounting prior to the soldering, thesolder facilitation layer including a metal that is not a component ofthe solder preparation layer on either the first contact surface or thesecond contact surface.
 13. The method of claim 12, wherein applying thesolder facilitation layer includes at least one of placing a sheet ofthe metal between the first contact surface and the second contactsurface prior to the soldering, and depositing a layer of the metal thatis not a component of the solder composition onto one or both of thefirst contact surface and the second contact surface prior to thesoldering.
 14. The method of claim 1, further comprising matching afirst thermal expansion characteristic of the carrier mounting to asecond thermal expansion characteristic of the semiconductor laser chip.15. A tunable laser-based trace gas analyzer, comprising: asemiconductor single-frequency laser chip including: a first contactsurface having a target surface roughness; a metallization layer of600·10⁻¹⁰ m of titanium applied to the first contact surface; a metallicdiffusion barrier layer applied to the metallization layer, wherein themetallic diffusion barrier layer includes multiple layers of differingmaterials; and a solder preparation layer applied to the metallicdiffusion barrier layer, wherein the solder preparation layer includesan approximately 2000 to 5000·10⁻¹⁰ m thickness of gold; and a carriermounting to which the laser chip is soldered along the first contactsurface of the laser chip using a solder composition, wherein the soldercomposition is heated to less than a threshold temperature at whichdissolution of the metallic diffusion barrier layer into the soldercomposition occurs, and wherein the metallic diffusion barrier layer iscontiguous such that the solder composition does not directly contactsemiconductor materials of the laser chip, such that there is no directpath by which constituents of any of the laser chip, the soldercomposition and the carrier mounting can diffuse across the metallicdiffusion barrier layer.
 16. The tunable laser-based trace gas analyzerof claim 15, wherein the tunable laser-based trace gas analyzer is atunable diode laser absorption spectrometer.
 17. The tunable laser-basedtrace gas analyzer of claim 15, wherein the metallic diffusion barrierlayer includes a first metallic diffusion barrier layer includingplatinum and a second metallic diffusion barrier layer underlaying thefirst metallic diffusion barrier layer, the second metallic diffusionbarrier layer including palladium, nickel, tungsten, molybdenum,titanium, tantalum, zirconium, cerium, gadolinium, chromium, manganese,aluminum, beryllium, or yttrium.
 18. The tunable laser-based trace gasanalyzer of claim 15, wherein the target surface roughness is less thanapproximately 100·10⁻¹⁰ m RMS, and/or wherein the target surfaceroughness is less than approximately 40·10⁻¹⁰ m RMS.
 19. The tunablelaser-based trace gas analyzer of claim 15, further comprising a solderfacilitation layer between the first contact surface and a secondcontact surface on the carrier mounting, the solder facilitation layerincluding a metal that is not a component of the solder preparationlayer on either the first contact surface or the second contact surface.20. The tunable laser-based trace gas analyzer of claim 15, furthercomprising another barrier layer applied to a second contact surface ofthe carrier mounting, wherein the laser chip is soldered to the carriermounting along the second contact surface.